A wide variety of methods have been proposed and explored for producing optical fibers. As optical fiber technology has matured, three main production methods, MCVD, VAD, and OVD have emerged. All involve the deposition of glass particulates (frequently referred to as “soot”) on a starting substrate, then consolidating the particulates into a solid glass body. The techniques involve producing the particulates using an in situ vapor phase reaction. The vapor phase reaction is induced using a torch, and the flame of the torch is directed at the starting substrate. In the MCVD method, the torch is directed on the outside of a glass starting tube, and the glass precursor gases are introduced into the interior of a glass tube. The particulates are deposited on the inside surface of the tube. In the VAD and OVD methods, the torch and precursor gases are directed onto the outside surface of a starting rod and the particulates are deposited on the end or side of the rod, respectively. Each technique is highly effective, and widely practiced. Each has well known advantages over the other.
For producing very high quality central core and inner cladding material, the MCVD process would appear ideal. In the MCVD technique, the particulate layer grows incrementally in a radial direction. Due to this incremental radial growth, MCVD is capable of producing more complex refractive index profiles than the VAD method. Complex index profiles are produced by changing the radial composition of the particulate layer for each feature of the profile. Additionally, complex index profiles frequently have one or more features with a depressed (relative to pure silica) index. Depressed index regions are usually formed by doping the particulates with fluorine. As will be described in more detail below, the inside tube deposition method (MCVD) is more suitable for fluorine doping than the either of the outside rod methods (VAD or OVD).
However, the need for using a starting tube can be a limiting factor in the MCVD method. One limitation is when the glass in the MCVD starting tube is not of sufficient quality and low loss for large, state of the art, preforms (where some fraction of the optical power would be carried by the starting tube material). If the initial tube quality limitation is avoided by the use of ultra pure (and typically expensive) material to fabricate the starting tube, the exposure of the tube to the oxy-hydrogen torch typically used in MCVD as a heat source may compromise the effective starting tube quality by the addition of hydroxyl ions to a significant tube depth. Finally, the desired refractive index profile may require a dopant level in the region provided by the starting tube glass that is not compatible with successful MCVD processing (viscosity, tube stability or heat transfer considerations).
In VAD methods, the silica soot deposits and grows axially from a starting bait rod. A significant advantage of the VAD technique is that it can be practiced as a continuous process. This allows in-line deposition, purification, drying, and sintering. After deposition is complete, the starting rod is separated from the deposited body and the entire preform, unlike conventional MCVD, may thus be made of CVD-deposited material. As a general proposition, VAD methods are effective and widely practiced, but they still do not match the ability of MCVD to control precisely the radial deposition of index changing dopants, and thus the radial refractive index profile. Because of this, VAD methods and other soot deposition/subsequent sintering methods such as Outside Vapor Deposition (OVD) are limited in the complexity of the fiber designs that can be efficiently produced. Moreover, the VAD method is not well adapted for fluorine doping. This is especially the case for in-line VAD processes.
The recognition in the prior art that in a single mode optical fiber the core and inner cladding together carry greater than 95% of the optical power but typically comprise less than 5% of the fiber mass, has resulted in manufacturing processes that focus special attention on the fabrication of this region. Methods have evolved in which the core and inner cladding regions of the preform are produced by a relatively advanced and expensive method, while the outer cladding, the bulk of the preform, is produced by a less demanding, less expensive, process. The integration of the core rod and the cladding is carried out in an overcladding process. The overcladding process is described generally in U.S. Pat. No. 6,105,396 (Glodis et al), and PCT/EPT00/02651 (25 Mar. 2000), which are incorporated herein by reference for details of the general techniques.
The overcladding process overcomes some of the limitations in the complexity of preforms produced by the VAD technique. Overcladding may involve multiple overcladding tubes, each adding a distinct cladding region, to reach the desired complexity of the fiber refractive index profile.
A commonly used process of this type is the so-called rod-in-tube method, where the core rod is made by a very high quality dopant-versatile process, and the cladding tube is made of less expensive, lower purity, glass. In some cases, glass with a single composition provides a low cost choice. In the rod-in-tube overcladding process, the core rod is inserted into the cladding tube, and the tube collapsed around the rod to form a unitary body. Again, multiple overcladding steps may be used, and in some cases one or more of the final overcladding steps may be combined with the fiber drawing operation.
If a tube overcladding process is used, suitable cladding tubes may be produced by soot deposition or extrusion of fused quartz. Making these very large cladding bodies with a soot based synthetic glass process leads to high quality glass but requires extensive processing and is relatively expensive. Large bodies of fused quartz are less expensive, but are generally not of sufficient purity.
In summary, the VAD method when combined with the rod-in-tube overcladding methods provides a rapid and economical method for forming large glass core rods with relatively simple index profiles. However, where the cladding comprises depressed index features, commercially available depressed index cladding tubes of the prior art do not provide the desired optical quality for the overall preform body.